In recent years, in line with an increase in the price of energy sources due to the depletion of fossil fuels and amplification of interests in environmental pollution, environmentally-friendly alternative energy sources has become an indispensable element for future life. Thus, research into various power generation technologies using nuclear power, solar power, wind power, tidal power, etc. has continuously conducted, and great interests in power storage devices for more efficiently using the energy thus generated have also grown.
In particular, as mobile devices have been continuously developed and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for such mobile devices. Accordingly, much research into batteries satisfying various needs has been carried out.
Typically, in terms of the shape of batteries, the demand for prismatic secondary batteries or pouch-shaped secondary batteries that are thin enough to be applied to products, such as cellular phones, is very high. In terms of the material for batteries, on the other hand, the demand for lithium secondary batteries, such as lithium ion batteries and lithium ion polymer batteries, which exhibit high energy density, discharge voltage, and output stability, is also very high.
In addition, the secondary battery may be classified based on the structure of an electrode assembly, which has a structure in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are stacked. Representative examples of such an electrode assembly include a jelly-roll type (winding type) electrode assembly in which long-sheet type positive electrodes and long-sheet type negative electrodes coated with an active material are wound in the state in which a separator is interposed between the positive electrode and the negative electrode, and a stack type (laminating type) electrode assembly in which a plurality of positive electrodes and negative electrodes, each of which are cut in units of a predetermined size, are sequentially stacked in the state in which a plurality of separators is interposed respectively between the positive electrodes and the negative electrodes. In recent years, in order to solve the problems with the jelly-roll type electrode assembly and the stack type electrode assembly, there has been developed a stacked/folded type electrode assembly, which is a combination of the jelly roll type electrode assembly and the stacked type electrode assembly, having an improved structure in which a predetermined number of positive electrodes and a predetermined number of negative electrodes are sequentially stacked in the state in which a predetermined number of separators are disposed respectively between the positive electrodes and the negative electrodes to constitute a unit cell, after which a plurality of unit cells is sequentially wound in the state of being placed on a separation film.
In addition, the secondary battery may be classified according to the shape of the battery into a cylindrical battery or a prismatic battery in which an electrode assembly is built in a cylindrical or rectangular metal container, and a pouch-typed battery in which an electrode assembly is built in a pouch-shaped case made of an aluminum laminate sheet.
In particular, in recent years, a pouch-typed battery having a structure in which a stack-typed or a stack/folding typed electrode assembly is built in a pouch-typed battery case made of an aluminum laminate sheet has attracted a great deal of attention due to low manufacturing cost, small weight, easy morphological deformation, etc., and its usage is also gradually increasing.
In general, such a secondary battery is manufactured by coating an electrode mixture comprising an electrode active material, a conductive agent, a binder, etc., onto an electrode current collector, and drying the coated collector to prepare an electrode, laminating the electrode together with a separator, enclosing the electrode together with an electrolytic solution in a battery case, and then sealing the case.
FIG. 1 is a schematic view showing a manufacturing process of a conventional positive electrode for a secondary battery.
Referring to FIG. 1, the positive electrode 100 is formed by a process in which the positive electrode mixture 110 including positive electrode active material particles 111 and binders 112 is coated in a liquid state onto a top surface of an positive electrode current collector 120, and then dried 130.
Here, in the drying process 130 for the positive electrode mixture 110, a solvent contained in the positive electrode mixture 110 is dried 130, and the binder 112 is contained in the positive electrode mixture 110 in a state of being dissolved in the solvent. Therefore, in the process of drying the solvent, the binder 112 dissolved in the solvent is transferred to the upper portion of the negative electrode mixture 110.
Accordingly, a relatively small amount of binder component is disposed between the positive electrode mixture 110 and the positive electrode current collector 120, thereby lowering the adhesive force between the positive electrode mixture 110 and the positive electrode current collector 120, and also increasing the resistance of the positive electrode 100. Therefore, there is a problem that the structural stability and electrical performance of the secondary battery including the positive electrode 100 are deteriorated.
Further, these problems act as factors that increase the fraction defective in the electrode manufacturing process, lower the reliability of the electrode manufacturing process, delay the overall process time and thus increase the manufacturing cost.
Therefore, there is a high need for a technology capable of fundamentally solving such a problem.